See:
Carrot (Daucus carota) - Alternaria Leaf Blight
Cause The fungus, Alternaria radicina (formerly Stemphylium radicinum), can be seedborne or may overwinter in crop refuse. A. radicina typically is responsible for most cases of black root rot under cool soil or storage conditions. A separate Alternaria species, A. carotiincultae, has been reported as pathogenic on carrot in other regions; it is closely related to A. radicina and it is difficult to distinguish between these two species based on morphological characteristics and colony appearance.
Symptoms In Oregon, black root rot typically occurs either in late winter or in early spring on overwintered carrots or carrots grown for seed production. The fungal pathogen attacks the leaf-stalk base and crown and may infect in the fall to incite late-winter and spring lesions. Symptoms may be small to extensive black lesions on the root surface, with a well-defined border between infected and healthy tissues. Such lesions may not penetrate deeply. Lesions may enlarge slowly in colder temperatures, even those near freezing. If lesions are extensive, plants may be stunted and also are prone to secondary infections by other organisms. Black root rot may occur on carrots in storage, but the decay does not appear to spread from root to root in storage. This fungus can cause damping-off of carrot seedlings. Under certain environmental conditions, A. radicina can cause a foliar blight similar to the Alternaria leaf blight incited by A. dauci.
Cultural control Together, these measures should keep the diseases under control.
- Use pathogen-free seed.
- Irrigate so that carrots are dry by nightfall.
- Rotation out of carrots for at least 1 year is important because refuse is the main way the organisms overwinter. Plow under crop debris soon after harvest.
- For seed crops, obtain transplant carrots free of Alternaria infection.
Chemical control Good eradication of A. radicina from carrot seed has been achieved with hot water (122°F for 30 min).
Seed treatments
- Nevado 4F at 1 pint/100 lb seed plus a dye. For Washington and Idaho only. 24-hr reentry.
If the disease is common, spraying may be necessary. Rovral applied in fall may prevent black rot on roots of seed carrots, although few data are available to verify this.
- Copper products. (Group M1)
- Bordeaux 6-6-100.
- C-O-C-S WDG at 1 to 1.9 lb/A. 48-hr reentry.
- Dithane F45 Rainshield (Group M3) at 1.5 quarts/A on 7- to 10-day intervals can be used on seed crops only. Washington and Oregon only (SLN WA-220008, SLN OR-240011). 24-hr reentry.
- Iprodione formulations (Group 2) are labeled.
- Iprodione 4L AG at 1 to 2 pints/A in a minimum of 10 gal/A water on 7- to 14-day intervals. Preharvest interval is 0 days. 24-hr reentry.
- Meteor at 1 to 2 pints/A in a minimum of 10 gal/A water on 7- to 14-day intervals. Preharvest interval is 0 days. 24-hr reentry.
- Rovral 4 Flowable at 1 to 2 pints/A in a minimum of 10 gal/A water on 7- to 14-day intervals. Preharvest interval is 0 days. 24-hr reentry.
- Roper DF Rainshield (Group M3) at 2 lb/A on 7- to 10-day intervals can be used on seed crops in Washington (SLN WA-1300003). 24-hr reentry.
References Farrar, J.J., Pryor, B.M., and Davis, R.M. 2004. Alternaria diseases of carrot. Plant Disease 88:776-784.
Park, M.S., Romanoski, C.E., and Pryor, B.M. 2008. A re-examination of the phylogenetic relationship between the causal agents of carrot black rot, Alternaria radicina and A. carotiincultae. Mycologia 100(3):511-527.
Pryor, B.M., Davis, R.M., and Gilbertson, R.L. 1994. Detection and eradication of Alternaria radacina on carrot seed. Plant Disease 78:452-456.